When driving a three-phase AC motor, an inverter device that converts a DC current supplied from a DC power supply into a three-phase (U-phase, V-phase, and W-phase) AC current performs a switching control (Pulse Width Modulation (PWM) control) to drive the motor. The inverter device includes a smoothing capacitor connected to the input of the inverter device in parallel with the DC power supply, which smoothes the input current.
When driving the three-phase AC motor, a switching loss is large in the three-phase modulation control in which the phases are constantly switched. A two-phase modulation control, in which any one of the phases is constantly non-switched, is used for reducing the switching loss. In the standard two-phase modulation control, the central time point of a period during which the switching elements of the two phases for which the switching is being performed are turned on coincides with the central time point of a period during which the switching elements of the two phases are turned off. For example, when the switching of the switching element of the W-phase is stopped and the switching of each of the switching elements of the U-phase and the V-phase is performed, the timing chart of the control voltage is as shown in FIG. 4A.
In the two-phase modulation control, at the starting and ending time points of a period (switching stop period) in which the voltage is maintained at a high potential level or a low potential level, the voltage value of each phase sharply changes. This is a factor of increasing a current ripple. In the two-phase modulation control, the change of the voltages in each phase in the case in which the switching is not performed at the time point of the peak of the voltage is different from that in the case in which the switching is not performed at the time point of the peak of the current as shown in FIGS. 5 and 6.
If the case in which the switching is not performed at the peak of the voltage in FIG. 5 is compared with the case in which the switching is not performed at the peak of the current in FIG. 6, the change ratio of the voltage at the starting and ending time points of the switching stop period is greater in the case in which the switching is not performed at the peak of the current than in the case in which the switching is not performed at the peak of the voltage. Accordingly, in the two-phase modulation control, the period in which the switching is not performed (switching stop period) is set at the time point of the peak of the current of each phase whenever possible.
In the inverter device of the two-phase modulation control for driving the three-phase AC motor, it has been proposed that the phase shift is performed on the central time point of the period in which one of the switching elements of the two phases for performing the PWM control is turned on or off by π (180 degrees) to reduce the ripple current generated in the smoothing capacitor as shown in FIG. 4B. Refer to Patent Document 1. The value π corresponds to a half value of the PWM carrier period 2π.